This invention relates to liquid crystal displays, or LCD type devices as they are sometimes called in the art.
In a typical liquid crystal display, which is a field-effect type device, electrical fields are created and adjusted to control the optical properties of one or more layers of liquid crystal material. The manner of making and using such devices is well-known, and examples thereof are contained in U.S. Pat. No. 3,600,060, issued Aug. 17, 1971, to D. Churchill; U.S. Pat. No. 3,654,606, issued Apr. 4, 1972, to F. J. Marlowe et al.; U.S. Pat. No. 3,881,809, issued May 6, 1975, to J. L. Fergason et al.; U.S. Pat. No. 3,918,796, issued Nov. 11, 1975, to J. L. Fergason; U.S. Pat. No. 4,039,252, issued Aug. 2, 1977, to F. Mizuno et al.; U.S. Pat. No. 4,068,923, issued Jan. 17, 1978, to P. Toida; U.S. Pat. No. 4,224,617, issued Sept. 23, 1980, to C. R. Stein; U.S. Pat. No. 4,335,936, issued June 22, 1982, to K. Nonomura et al.; and U.S. Pat. No. 4,385,806, issued May 31, 1983, to J. L. Fergason.
LCDs are generally best viewed or read from particular directions or angles with respect to the display construction, as noted in U.S. Pat. No. 4,385,806. From some angles, there will be an insufficient apparent optical contrast between energized segments of the display and the display background. That is, the displayed symbols or characters will appear blurred or be difficult to discern.
The set or collection of angles or viewing lines from which the display may be effectively viewed defines what is here called the "apparent viewing cone". For a given display, the apparent viewing cone will have a specific shape and a specific orientation, and these will be characteristics of that particular display. A display may be viewed efficiently from angles within the apparent viewing cone, but not from other angles.
It is known that one can alter the orientation of the apparent viewing cone, without necessarily altering the apparent viewing cone shape, by increasing or decreasing the magnitude of the electric field by which the display elements are fully activated or energized. Thus, in manufactured display circuits there is often a potentiometer or similar component by which the energizing signal levels may be sometimes adjusted.
It is known to activate the display segments using an energizing signal which is purely a D.C. voltage. It is also known that in most displays it is not critical or important whether the energizing signal has a positive or a negative voltage potential. Thus, it has been understood that display segments may be activated using an energizing signal which is an A.C. voltage signal, such as an A.C. squarewave voltage signal, as has been used in a number of prior displays.
In U.S. Pat. No. 3,654,606, it is said that the operating life of the liquid crystal material can be extended by employing alternating voltage excitation rather than direct voltage excitation, and a circuit for providing an A.C. voltage energizing signal for an LCD is disclosed.
In U.S. Pat. No. 4,224,617, an LCD is disclosed in which the display electrodes are driven by sinusoidal or square waveforms of multiple frequencies and phases in a manner such that the electrode leads are maintained essentially invisible during the display operation.
In U.S. Pat. No. 4,335,936, a dot matrix type LCD is disclosed in which the electrode leads are common to multiple segments which may be individually activated by driving the electrodes with properly-timed squarewave voltage signals having multiple amplitudes. This type of driving method is sometimes called "multiplexing".
In order to activate the display segments in an LCD, the energizing signal conventionally has an "activation signal portion". The absolute value of the magnitude of this activation signal portion by definition exceeds the minimum voltage threshold level needed to cause the display segments to become activated or change optical states.
In U.S. Pat. No. 4,385,806 means are disclosed for improving the response time of liquid crystal displays in which the liquid crystal layer is relatively thick. The display electrodes are continuously biased by a D.C. voltage signal which is just below the minimum voltage threshold level needed to energize the display. A high frequency A.C. voltage signal is then superimposed upon the D.C. voltage biasing signal in order to produce a combined signal which energizes the display segments.
For purposes of additional background and disclosure, the specifications and drawings in all of the patents cited above are hereby incorporated by reference as if set forth fully herein.